A shock absorber attenuates vibration applied to a vehicle, an apparatus, a structure, and the like. JP 2007-225066A discloses a shock absorber D3 for a front fork that suspends a front wheel in a saddled vehicle such as a two-wheeled or three-wheeled vehicle as illustrated in FIG. 8.
The shock absorber D3 has a shock absorber main body T including an outer tube T1 connected to a chassis side and an inner tube T2 connected to a wheel side. The shock absorber D3 contracts or expands as the inner tube T2 is inserted into or retracted from the outer tube T1.
The shock absorber D3 includes: a cap member (support member) 7 that stops a chassis-side opening of the shock absorber main body T; a piston rod (shaft member) 2 supported by the cap member 7 and retractably inserted into the inner tube T2; an annular piston (partitioning wall) 3 held in the outer circumference of the leading end of the piston rod 2 to make sliding contact with an inner circumferential surface of the inner tube T2; and an expansion-side chamber (chamber in one side) L1 and a compression-side chamber (chamber in the other side) L2 that are partitioned by the piston 3 and contain a working fluid.
The expansion-side chamber L1 communicates with the compression-side chamber L2 through three fluid paths, including an expansion-side piston fluid path (not illustrated) formed in the piston (partitioning wall) 3, a compression-side piston fluid path 31 formed in the piston (partitioning wall) 3, and a bypass path B7 formed in the piston rod (shaft member) 2. Each fluid path is provided with each valve (such as an expansion-side damping valve V13, a compression-side damping valve V14, and an orifice valve V15).
The expansion-side piston fluid path (not illustrated) is provided with the expansion-side damping valve V13. The expansion-side damping valve V13 only allows the working fluid to pass through the expansion-side piston fluid path and move from the expansion-side chamber L1 to the compression-side chamber L2 and exerts predetermined resistance to the working fluid passing through the expansion-side piston fluid path.
The compression-side piston fluid path 31 is provided with the compression-side damping valve V14. The compression-side damping valve V14 only allows the working fluid to pass through the compression-side piston fluid path 31 and move from the compression-side chamber L2 to the expansion-side chamber L1 and exerts predetermined resistance to the working fluid passing through the compression-side piston fluid path 31. In addition, the compression-side damping valve V14 includes: a valve body V14a having a ring plate shape whose outer circumference seats on a seat surface formed in the expansion-side chamber side (upper side in FIG. 8) of the piston 3; a bias spring V14b that biases the valve body V14a toward a closing direction (piston side); and an adjuster V14c that adjusts an elastic force of the bias spring V14b. 
The bypass path B7 formed in the piston rod 2 is provided with an orifice valve V15. The orifice valve V15 includes a needle-like valve body V15a that forms an orifice by inserting a leading end thereof into the bypass path B7 to narrow the bypass path B7 and an adjuster V15b that changes a fluid path area of the bypass path B7 by driving the valve body 15a. 
When a piston velocity is in a low velocity range during contraction or expansion of the shock absorber D3, a working fluid in one of the expansion-side chamber L1 and the compression-side chamber L2 compressed by the piston 3 passes through the orifice formed by the orifice valve V15 and the bypass path B7 and moves to the other one of the expansion-side chamber L1 and the compression-side chamber L2. As a result, the shock absorber D3 generates a damping force caused by the resistance of the orifice valve V15. This damping force can be adjusted by the adjuster V15b of the orifice valve V15.
When the piston velocity is in a middle or high velocity range during expansion of the shock absorber D3, the working fluid of the expansion-side chamber L1 compressed by the piston 3 opens the expansion-side damping valve V13 and moves to the compression-side chamber L2 through the expansion-side piston fluid path (not illustrated). As a result, the shock absorber D3 generates an expansion-side damping force caused by the resistance of the expansion-side damping valve V13.
When the piston velocity is in a middle or high velocity range during contraction of the shock absorber D3, the working fluid of the compression-side chamber L2 compressed by the piston 3 opens the compression-side damping valve V14 and moves to the expansion-side chamber L1 through the compression-side piston fluid path 31. As a result, the shock absorber D3 generates a compression-side damping force caused by the resistance of the compression-side damping valve V14. This compression-side damping force can be adjusted by the adjuster V14c of the compression-side damping valve V14.